Go through the list of the nearest stars, and binaries abound…but few are the close binaries needed for the iconic twin sunset scenes from classic sci-fi. Gliese 866, better known by its variable-star designation EZ Aquarii, is the nearest possibility: located 11.1 light-years from Earth, it’s actually a trinary system, with a third red-dwarf sun orbiting a central pair of red-dwarf suns that whiz about each other close enough to loom large and close together in the sky of another Earth. To wit, the central pair orbit each other every 3.786 days with a separation of 0.03 AU. The brighter of the two is 11.87% as massive as our sun, shining with 0.078% of our sun’s luminosity; the dimmer of the two weighs in at 9.3% of our sun’s mass, putting out 0.012% our sun’s luminosity. Combined they shine 0.09% as bright as our sun. Dim, but get close enough and they start to put out a lot of light and heat onto a planet. How close?
Consider that at a separation of 0.03 AU, a stable orbit around both these stars (i.e. a “circumbinary orbit”) would be no closer than about 0.1 AU, using the rule of thumb that orbits beyond 2-3 times the binary separation are typically stable. From the point of view of a planet orbiting at 0.1 AU, the brighter of the two suns spans 29 arcminutes across the sky (comparable to our sun or moon!) and the dimmer and redder or the two spans 17 arcminutes across the sky. Assuming the planet’s orbit was coplanar, the stars would seem to dance around each other: reaching a maximum apparent separation of perhaps 10 degrees in the sky, they would then eclipse each other and separate again, with a full cycle taking of course 3.8 Earth days.
All very elegant…but even with the orbit being as close as it could be and be a circumbinary planet, the luminosity is still too low to support life as we know it: Earth’s equilibrium temperature is 255 kelvin; this world’s, assuming an Earth-like albedo of 0.3, would be 139 kelvin. Oops. The source for this is my own handy planetary equilibrium temperature calculator, by the way. 139 kelvin equals -209 degrees Fahrenheit, so even with a decent greenhouse effect and a dark color it’s going to be one cold world. But all is not lost.
In my sci-fi universe water worlds are old stuff anyway once you’ve reached 11 light-years distance from Earth. What about something more exotic? Are there substances that would form oceans at 139 kelvin or thereabouts?
Turns out: yes! Methane is often proposed as an alternative solvent for life, and is actually even found in our own solar system in the form of Titan’s lakes. At one Earth atmosphere of pressure, methane’s melting point is 91 kelvin, with its boiling point being 112 kelvins. So this would might actually be too warm…however, methane’s boiling point (like water’s) depends strongly on pressure. At 10 atmospheres the boiling point rises to 149 kelvin, which is within range for our world. At 20 atmospheres the boiling point rises to 166 kelvin. At 30 atmospheres we’re talking about 178 kelvin. So under pressures greater than Earth but well within what we could envisage (Venus, after all, has over 90 atmospheres…), methane would stably exist as a liquid.
This could even extend, unlike Titan, to global seas of liquid methane, possibly quite deep, forming the methane equivalent of a water world. I was inspired to build out a water (well…liquid) world because EZ Aquarii, as the name suggests, is in the constellation of Aquarius. Not that it makes water more likely in real life versus any other constellation — constellations are, after all, just patterns in the sky as viewed from Earth — but it is a nice theme. But the abundance of water worlds in my previous worldbuilding inspires me to put a twist on the “water world” concept.
Titan of course provides precedent for the general concept, but as far as getting the spectacular twin sunset effect, it imposes a bit of a problem in the form of the infamous haze that characterizes the lower atmosphere, hence why Titan’s surface, even under one bar or so of pressure, is dominated by an orange smoggy sky out of which you can’t see anything…like stars or suns. Oops.
But there might be a way around this: consider that Titan’s atmosphere, in addition to the 5% or so that’s methane, is 95% nitrogen. And it’s an ultraviolet-radiation-mediated reaction with the nitrogen that causes all the haze. Remove the nitrogen and the planet’s atmosphere would be pretty clear.
Of course remove the nitrogen and the atmosphere would only be 5% as thick as it is today, and that much less capable of supporting liquid on the surface (remember, lower pressure means a tighter liquid range, and vice versa). However, let’s suppose that our planet around the twin suns is much more massive than Titan, perhaps even more massive than the Earth; in addition to nitrogen and methane, it would be capable of holding onto carbon dioxide and carbon monoxide against atmospheric escape, and even small amounts of helium if it were in the super-Earth range.
All these gases are vented out from the planet’s interior via volcanism and so forth…and will tend to be retained by more massive and/or colder bodies, which is why Titan, Earth, and Venus all have a lot of nitrogen; even Triton has a wisp of it.
Let’s suppose that our planet started with a very thick atmosphere that was predominately nitrogen; perhaps there was vigorous volcanism. Or maybe a “super-Titan” just ends up this way; after all, Titan already has almost an entire Earth atmosphere’s worth of the stuff now, so a bigger version might have an atmosphere that was still thicker. What you end up with of course is even thicker haze…at first.
But suppose that life evolves on this planet whose metabolic processes draw down nitrogen and sequester it in non-gaseous forms, not too unlike the nitrogen-fixing bacteria of Earth but on a far larger scale. Over billions of years, the atmospheric nitrogen would be drawn down to a very low level, dozens of bars worth of it locked up in salts in the ground, leaving a much thinner atmosphere made up of what was left of the other components.
What would that look like? Carbon dioxide is the other main volcanic gas, but at these temperatures it would snow out and be frozen, so it wouldn’t be a major player in the air. Methane is this world’s version of water vapor: it’s not going to be much more than a trace gas. So what does that leave us? Carbon monoxide, essentially, is the most major remaining gas. Like carbon dioxide it’s produced by volcanism, and its freezing point is considerably lower than carbon dioxide’s. At 139 kelvin and 30 atmospheres of pressure it would be well within the gaseous range.
Carbon monoxide is a rather clear gas, transparent in the visible part of the spectrum, much like nitrogen and oxygen are; so you see the sun, the stars, and the clouds. The primary difference visually is that the visible “window” shifts toward the near-infrared; from a human perspective, blue light is absorbed but red light is scattered, with the net effect of the sky being a bright orange-red. Very alien, and very classic sci-fi.
Methane, meanwhile, is not terribly different from water, in as much as it forms clear to white ice and snow, as well as clear droplets. The net effect is that the clouds billowing up in this carbon monoxide atmosphere and its red sky would look very familiar. Convection would make clouds puff, lightning would belch out, and rain would fall in shafts over the sea. There would even be rainbows: methane droplets would create an arc 38 degrees in radius from the antisolar point rather than the familiar 42, but all the same colors we’re familiar with would be visible.
The seas themselves wouldn’t have nearly as much spray or foam as we’re familiar with, and would appear dark and mysterious: liquid methane absorbs visible light strongly, and so a huge volume of it like an ocean would appear more or less black. So while the rains and clouds would look familiar to human explorers, the sky is red, and the ocean is an opaque black, smoothly-textured even in the face of large waves to the extent it would feel alien, even if what you see from horizon to horizon looks like the ocean.
Very cool. Of course the nature of the suns imposes some more alien qualities: the twin suns are very red — in fact they’re so deep into the red dwarf range that they approach the lower limit of hydrogen-burning stardom altogether — so the ambient lighting will be much more reddish than we’re accustomed to. This doesn’t mean it’ll be monochrome, but it does mean it’ll be similar to being under a very warm incandescent light bulb in terms of the coloring. The net effect is that rainbows will appear much brighter in the red part of the visible spectrum, whereas the bluer parts of the spectrum will be much weaker (much like is the case deep into sunset on Earth, only here it’s permanent, with sunset being even redder still on this world than any Earth sunset, culminating in a deep crimson). The sky of course will be even redder than would have been the case otherwise, with the white clouds still feeling white but a warmer white, again much like a sheet of paper being illuminated by an incandescent light.
The world is close enough to both suns, and their separation grows wide enough, that shadows will appear to be doubled most of the time (except when the suns are eclipsing or approach very closely, in which case there are more familiar single shadows). One might also expect that this world would be tidally locked, albeit interestingly to the solar system barycenter rather than one of the stars or the other…and you would be correct.
But, and this is where it really gets fancy, the nature of tidal locking depends on the planet’s orbital eccentricity (i.e. how elliptical the orbit is). Beyond a certain eccentricity a 1:1 tidal lock is impossible, and a different (“higher-order”) spin-orbit resonance is in play instead. Mercury is an example from our own solar system: too eccentric for a 1:1 tidal lock, instead it’s in a 3:2 spin-orbit resonance; it rotates 3 times for every 2 orbits. The net effect is an apparent solar day is exactly equal to two local years. It also leads to interesting effects like the sun rising, then falling, then rising again as the planet moves along its orbit.
Planets in close-in orbits tend to drift into paths circular enough to permit a 1:1 tidal lock with time…but the presence of other bodies can pump eccentricity over time. The third star in the system, orbiting much further out with a period of 822 days, could easily provide enough perturbation to keep our planet’s orbit nice and eccentric around the central pair. How eccentric?
While a Mercury-style 3:2 spin-orbit resonance is the most conservative option, a 2:1 spin-orbit resonance would be much more interesting, and very plausible if the planet’s eccentricity is somewhat greater than Mercury’s. Like Mercury, this would lead to the twin suns rising, falling, and then rising again as their apparent sizes grow and shrink with the seasons. A cool effect. It also leads to this world enjoying a normal cycle of day and night across the whole planet. At 0.1 AU, the orbital period around the twin suns would be 25 days; with a 2:1 spin-orbit resonance (the planet spins twice for every orbit) the net effect is a solar day lasts exactly as long as a solar year. So there would be 25 days of light and 25 days of darkness (roughly; the exact timing depends on the motion of the suns around the common barycenter as well as the orbital effects like is seen on Mercury, potentially leading to the suns stalling in their procession through the sky or even rising and setting multiple times, but it should be approximately correct).
That’s one long day-night cycle, but an atmosphere 30 bars thick would efficiently redistribute heat, potentially leading to the daily range in temperatures being limited, though the thick air coupled with the heating pressures and the abundance of “moisture” from the methane ocean would lead to intense storms, heavy rains, and strong waves. Slow rotation should lead to large storms that cover much of the planet in streaks of cloud whose centers of circulation move only very slowly, a lot like a whole planet’s worth of “cut-off lows”, to use the term from Earth meteorology.
Though I wonder about “super-rotation”, as astronomers put it; with this much energy coursing through the atmosphere and the air being so thick and having such verticality to it, the upper layers might be stimulated to rotate much faster than the surface does, as we see on Venus, which rotates only once every 243 days but whose cloudtops make a full circuit of the planet once every 4 days.
Naturally with air that has this much verticality to it, the shear would be intense, potentially leading to horizontal vortices of methane cloud that would rotate over long distances and across wide reaches of altitude, much like horizontally-oriented versions of Earth’s tornadoes; such storms might actually occur on Venus for similar reasons.
Only unlike Venus the conditions here would be much more clement both for human colonists and for local life; yes, it’s 200 degrees below zero in Fahrenheit terms but that’s not really all that much colder than parts of Antarctica. Quite comparable to Saturn’s cloudtops in fact. So human explorers would find hardcore arctic gear and breathing masks more than sufficient, at least at the 1 bar level many miles up above the surface, where sky cities and floating habitats could be constructed. A full spacesuit would certainly not be required to live there.
The surface is another problem; since the greenhouse effect is minimal it wouldn’t be that much warmer than the human-habitable layer, but at 30 atmospheres you’re above the limit of what humans can ever withstand without a pressure suit. The absolute limit with pressurized breathing mixes is thought to be around 20 times Earth atmospheric pressure, with oxygen at Earth-like partial pressure but with the other 19 bars or so worth of pressure coming from inert helium (“heliox” air mixtures, they’re called). Helium does not cause narcosis (i.e. “the bends”), unlike nitrogen, but high-pressure nervous syndrome is unavoidable and becomes quite acute at around the 20 atmosphere mark. Adaptation might be feasible, but it’d be difficult in any event.
Still, if the atmosphere was not quite at 30 bars, but at more like 20, or if a way could be found to push the envelope as high as 30 bars, the surface would be a spectacular place to visit. You wouldn’t need a spacesuit; yes, you’d be fighting off high-pressure nervous syndrome, but with a hardcore heated wetsuit and a breathing mask for oxygen, you could sail a sea of liquid methane with a clear red sky before you at midday, the light from the twin suns glinting off the obsidian-black liquid being whipped up in oddly foamless huge waves, clouds and rains and winds before you that look and even feel Earth-like. The wind and the sounds would be heavy in the thick air, but otherwise it would be so much like being on another Earth. Very classic sci-fi.
Does that extend to life being present in these methane seas? I’d say it does. While information on EZ Aquarii’s age is hard to come by, let’s say it’s on the order of billions of years old. Assuming vigorous geological activity, there’s no reason why this world wouldn’t have methane equivalents of hydrothermal vents and so forth to serve as primordial habitats for microbes. Metabolism would likely have to be anaerobic, but there’s no particular reason why an entire biosphere couldn’t have evolved in the fullness of time, complete with plankton, simple animals, and finally more complex organisms native to the seas.
Since we’re going for a classic sci-fi alien world vibe, why not posit long, thin, transparent eel-like creatures being the dominant fauna in these oceans? Such a coloring as transparent may well be advantageous in these black seas…though to them they wouldn’t experience the seas as being black. In the near-infrared liquid methane is as clear as water is in our part of the spectrum, so most likely you’d see the same sorts of vision, complete with colors, that we see on Earth, only shifted into the infrared. To them, interestingly, our seas of water would look black and opaque (water, famously, appears black in near-infrared cameras), which is an intriguing symmetry between human and alien. And no doubt to them what appears to us to be a red sky would look blue-ish, since the red part of our visible spectrum would be the shortest wavelengths their eyes would likely be sensitive to.
Anyway, with near-infrared light penetrating deep down into methane oceans, and the suns putting out most of their energy in the near-infrared anyway (remember these are low-mass red dwarfs), plankton sensitive to the near-infrared (perhaps even the selfsame lineages that drew down the nitrogen from the primordial atmosphere as part of their metabolic processes) would find it a very friendly environment. Many varieties of creatures could roam these seas with these photosynthetic plankton as the base of the food web, culminating with titanic-sized filter-feeders that would take the same role on this world as whales do on Earth. Perhaps they’d be transparent, perhaps they’d drink methane as opposed to water, but superficially they might look similar…especially when their tails flop in the distant roiling seas (tails are dictated by hydrodynamics, and methane’s characteristics aren’t too different from water, so most likely their body shape would be fairly similar; convergent evolution at its finest).
That’s already alien enough, but the skies might also be full of complex life-forms, albeit not as vigorous as a world like Thalassa (my take on Proxima Centauri’s planet) where the atmosphere is rich with energetic oxygen to use as metabolism. Most likely photosynthesizing floaters will evolve out of the seas and take to the skies, being the primary form of life in the clouds and skies of this world. Think something like kites or sky lanterns, only biological. On the dreamy end, but still very classic sci-fi, in keeping with the vibe I’m building here.
The ultimate finishing touch comes from the aurora. Yes, this world gets lights dancing in the sky too, just like Earth, only unlike Earth auroral outbursts are likely to be much more intense and frequent, owing to the red dwarfs it’s orbiting being flare stars (remember: the central pair carry a variable star designation for a reason). During solar storms the global magnetic field will buckle and aurora will dance through the sky globally, potentially bright enough during stronger storms to turn night into day on the dark side.
An interesting effect is since the atmosphere is primarily carbon monoxide, you’d see colors that might be different from anything in our solar system; carbon monoxide leads to red auroras deeper down, with middle to upper levels of the atmosphere being more purplish. So once again there is even more illumination of a reddish nature.
The auroras are bright enough that frankly photosynthesis based on the spectrum of light they put out may well be a secondary metabolic pathway for plankton, algae, and the like in these alien seas; this is especially likely to evolve as an adaptation to those long 25-Earth-day nights, when otherwise the plant life would have to hibernate. So the net effect is when the aurora come out, there might be a sudden bloom of plankton, algae, and other plant life worldwide, with the most spectacular effects being on the night side, where the black sea might turn to a reddish glow as a “superbloom” emerges…followed by a feeding frenzy from these whale-like creatures.
The alien nature of this scene is underlined by how there may well be the third sun shining on the dark side; the level of illumination it puts out would be comparable to the Moon on Earth, so you have a reddish light that’s easily enough to cast shadows and see by even without aurora or bioluminescence. But with all that going on, right after the central suns erupt into a solar storm? It would be one of the most spectacularly alien nights in our corner of the galaxy, straight out of a science-fiction storybook. An iconic moment in the early days of exploration might come when an adapted adventurer is on his sailboat and experiences such a night, and watches the twin suns rise in tandem at the horizon, their crimson light flickering from the sea and illuminating the transparent whale-like tails as they flop and feed, auroras diminished from their midnight peak but still far more spectacular than anything on Earth, the distant third sun still visible near the horizon as it prepares to set.
So spectacular that calling the place “EZ Aquarii” or “Gliese 866” doesn’t seem to do it justice. I’ve tossed around a few candidates inspired by Greek mythology, but “Xenothalassa”, from the words for “strange sea”, might just be the one that sticks in my sci-fi universe. Consider that “Thalassa” is the name for Proxima Centauri’s ocean planet, the first such body human explorers encounter on their journeys outward from our solar system, and this world orbiting EZ Aquarii would be another landmark in the history of exploring pelagian planets, only with a foreign liquid making up the global ocean
And strange it is indeed, because carbon monoxide sorta comes out of left field as a primary atmospheric constituent; very little work has been done on it, yet for a world belching out the usual volcanic gases in quantity only to have the nitrogen sequestered into and the carbon dioxide frozen and snowed out to the bottom of the sea, forming miles-thick layers of sediment (primarily ammonium salts, but in as much as they’re non-soluble in methane it would basically function the same as bedrock), it’s very plausible.
Object lesson: impose a few constraints (like working with a real system), make sure your world has a stable thalassogen, turn a few dials (hey what if this or that constituent of the likely primordial atmosphere was removed by life?), and you never know what you’ll find out. In real life I’m sure there are more worlds like this out there than we might think, that are physically plausible step by step but boggle our mind compared to what we see from our own solar system, but our powers of observation are limited; for the moment, we must rely on imagination…